1. Field of the Invention
The present invention relates to a control apparatus, a control method, and an engine control unit which calculate a control input to a controlled object based on a ΔΣ modulation algorithm or the like to converge the output of the controlled object to a target value.
2. Description of the Prior Art
Conventionally, a control apparatus of the type mentioned above is known, for example, from Laid-open Japanese Patent Application No. 2001-154704. This control apparatus comprises detecting means for detecting an output of a controlled object to output the result of detection as a detection signal indicative of a detected analog amount; deviation calculating means for calculating a deviation of the detection signal from a target value of an analog amount inputted from a higher rank apparatus; converting means for converting the calculated deviation to a 1-bit digital signal; and compensating means for compensating the 1-bit digital signal from the converting means to output the compensated signal as a manipulation signal (see FIG. 6 of the application).
In this control apparatus, the deviation calculating means calculates a deviation of a detection signal from a target value (analog amount) which is converted to a 1-bit digital signal by a ΔΣ modulation in the converting means. The converted signal is further compensated by the compensating means before it is inputted to a controlled object as a manipulation signal. In the foregoing configuration, the manipulation amount is generated in the opposite phase to the deviation so as to cancel the deviation of the output of the controlled object from the target value, and inputted to the controlled object. As a result, the output of the controlled object is controlled in feedback to converge to the target value.
According to the conventional control apparatus mentioned above, when a dynamic characteristic of a controlled object has a relatively large phase delay, a dead time, or the like, this causes a delay in outputting an output signal, which reflects an input signal from the controlled object, after the controlled object is fed with the input signal, leading to a slippage in control timing between the input and output of the controlled object. As a result, a control system could lose the stability. For example, when an internal combustion engine is controlled for an air/fuel ratio of exhaust gases using a fuel injection amount of the internal combustion engine as an input, a time lag is needed until the air/fuel ratio of the exhaust gases actually change after a fuel has been actually injected, so that the air/fuel ratio control experiences lower stability and controllability, resulting in an instable characteristic of exhaust gases purified by a catalyst.